Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery, which is used for a mobile device such as a mobile phone, is available as a commercial product. In general, the non-aqueous electrolyte secondary battery has a constitution in which a positive electrode and a negative electrode are connected to each other through an electrolyte layer; in the positive electrode, a current collector is coated with a positive electrode active material or the like, in the negative electrode, a current collector is coated with a negative electrode active material or the like, and in the electrolyte layer, a non-aqueous electrolytic solution or a non-aqueous electrolyte gel is held within a separator. According to occlusion and release of ions such as lithium ions in and from the electrode active material, a charge-discharge reaction of a battery occurs.
In recent years, it is desired to reduce an amount of carbon dioxide in order to cope with the global warming. A non-aqueous electrolyte secondary battery having a small environmental burden has been used not only for a mobile device or the like but also for a power supply device of an electric vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), and a fuel cell vehicle.
The non-aqueous electrolyte secondary battery for application to an electric vehicle is required to have a high output and a high capacity. As a negative electrode active material used for a negative electrode of a non-aqueous electrolyte secondary battery for an electric vehicle, a material having a high output and a high capacity is required from a viewpoint of a capacity and an output characteristic.
For example, WO 2002/059040 A describes an artificial graphite particle having a secondary particle structure in which a plurality of primary particles are assembled or bonded to each other, and the plurality of primary particles each having a layer structure in which an edge portion of each of the primary particles is bent into a polygonal shape are assembled or bonded to each other.
However, the invention described in WO 2002/059040 A does not specify the shape of the artificial graphite particle (primary particle and secondary particle structure) which is a negative electrode active material. Actually, the shape of the artificial graphite particle (primary particle and secondary particle structure) in WO 2002/059040 A is a nonuniform shape having many protrusions (irregular shape). Meanwhile, a coating film is formed on a particle surface of the negative electrode active material during charging and discharging. The shape of the negative electrode active material has an influence on formation of the coating on the particle surface of the negative electrode active material. Uniformity of the coating is involved in uniformity of a chemical reaction in the particle surface of the negative electrode active material. The low uniformity locally deteriorates the negative electrode active material during charging and discharging to reduce a charge-discharge capacity. Therefore, in the nonuniform shape having many protrusions (irregular shape) as in WO 2002/059040 A, the uniformity of the coating is low. Therefore, the negative electrode active material is locally deteriorated during charging and discharging to reduce the charge-discharge capacity. That is, in the nonuniform shape having many protrusions, uniformity of the coating is extremely low in the protrusions and therearound, and a region in which the coating is not formed tends to be present in the protrusions and therearound. In addition, the surface area of the protrusions (reaction area or contact area) is very large, and a reaction with an electrolyte is locally promoted rapidly during charging and discharging. At this time, the local reaction with an electrolyte proceeds significantly in the protrusions in the region in which the coating is not formed and therearound. As a result, in the protrusions, local deterioration of the active material (for example, inactivated due to a distortion, collapse, or the like of a crystal structure) proceeds rapidly due to repeated charging and discharging, and the capacity retention ratio of the secondary battery is reduced disadvantageously.